Use relative links for commits, mentions, and issues in markdown (#29427)

Fixes #29404

Use relative links for
- commits
- mentions
- issues

---------

Co-authored-by: silverwind <me@silverwind.io>

routers/common/markup.go

@@ -34,7 +34,8 @@ func RenderMarkup(ctx *context.Base, repo *context.Repository, mode, text, urlPr
 		if err := markdown.RenderRaw(&markup.RenderContext{
 			Ctx: ctx,
 			Links: markup.Links{
-				Base: urlPrefix,
+				AbsolutePrefix: true,
+				Base:           urlPrefix,
 			},
 		}, strings.NewReader(text), ctx.Resp); err != nil {
 			ctx.Error(http.StatusInternalServerError, err.Error())
@@ -79,7 +80,8 @@ func RenderMarkup(ctx *context.Base, repo *context.Repository, mode, text, urlPr
 	if err := markup.Render(&markup.RenderContext{
 		Ctx: ctx,
 		Links: markup.Links{
-			Base: urlPrefix,
+			AbsolutePrefix: true,
+			Base:           urlPrefix,
 		},
 		Metas:        meta,
 		IsWiki:       wiki,